1. Field of the Invention
The present invention relates to a method of driving a transistor and a device including a transistor driven by the method. The driving method and the device including a transistor driven by the method according to the present invention are particularly suitable for a light-emitting display including pixels arranged in a matrix, each pixel including a light-emitting element and a driving circuit for supplying current to the light-emitting element, and a method of driving the display. For example, an organic electroluminescent (hereinafter, abbreviated to “EL”) element is suitably used as a light-emitting element.
2. Description of the Related Art
The research and development of organic EL displays including organic EL elements as light-emitting elements have progressed in recent years. A typical organic EL display is of the active matrix (hereinafter, referred to as “AM”) type in which each pixel includes a driving circuit in order to extend the life of the organic EL element and achieve high image quality. The driving circuit includes a thin film transistor (hereinafter, also abbreviated to “TFT”) formed on, for example, a glass or plastic substrate.
In the organic EL display, a portion including the substrate and the driving circuit is called a “backplane”. As for a channel material of the TFT for the organic EL display backplane, for example, amorphous silicon (hereinafter, abbreviated to “a-Si”) and polycrystalline silicon (hereinafter, abbreviated to “p-Si”) have been studied. In addition, amorphous oxide semiconductor (hereinafter, abbreviated to “AOS”) has been newly proposed. AOS materials include, for example, indium (In) gallium (Ga) zinc (Zn) oxide (amorphous In—Ga—Zn—O; hereinafter, abbreviated to “a-IGZO”) and zinc (Zn) indium (In) oxide (amorphous Zn—In—O; hereinafter, abbreviated to “a-ZIO”). An AOS TFT has great potential as a TFT for a display backplane because the AOS TFT has a mobility more than ten times as high as that of an a-Si TFT and exhibits high uniformity due to its amorphous nature. A TFT including a channel layer of AOS is disclosed in, for example, Nomura et al., “Nature”, Vol. 432, pp. 488-492, 2004 and Yabuta et al., APL, 89, 112123, 2006.
Challenges to the achievement of high quality display in an AM organic EL display include, for example, (1) a change over time in the voltage-brightness characteristic of an organic EL element, (2) variations in the characteristics of TFTs, each serving as a component of a driving circuit, and (3) changes in characteristics of a TFT caused by electrical stress.
When an AOS TFT is included in each driving circuit, the above-described challenges (1) and (2) can be solved because the uniformity of the AOS TFT is high and a driving circuit for controlling current supplied from the AOS TFT to the organic EL element is used.
However, the above-described challenge (3) is left because there are changes in characteristics of the AOS TFT caused by electrical stress, mainly, the change of threshold voltage.